A lithium ion secondary battery has been widely used for portable electronic device such as mobile phones and notebook personal computers. For a cathode active material for a lithium ion secondary battery, a composite oxide of lithium and a transition metal or the like (hereinafter sometimes referred to as lithium-containing composite oxide) such as LiCoO2, LiNiO2, LiNi0.8Co0.2O2 or LiMn2O4 has been used.
Further, in recent years, for a lithium ion secondary battery for portable electronic device or for vehicles, downsizing and weight saving are required, and further improvement in the charge and discharge efficiency has been desired.
Patent Document 1 discloses a process of bringing an aqueous solution containing a specific cation M, an aqueous solution containing an anion N which reacts with the cation M to form a hardly soluble salt, and a lithium-containing composite oxide containing Li element and a transition metal element into contact with one another, followed by heating to provide a covering layer on the lithium-containing composite oxide, whereby the retention of the discharge capacity after a discharge cycle is repeated (cycle retention) is improved.
Further, Patent Document 2 discloses that the initial charge and discharge efficiency (initial efficiency) is improved by treating a powder of a lithium-containing composite oxide containing Li element and a transition metal element with nitric acid, treating it with an ammonia gas and then subjecting it to a heat treatment.